The delayed reciter K channel (lKr) in the heart plays an essential role in the membrane repolarization during the plateau phase of the cardiac action potential (AP). The objective of this proposal is to characterize the structure of the human IKr channel and to identify regions of the channel that are responsible for the properties of rapid inactivation and rectification. We propose that the human IKr channel will show enough sequence homology to the RK family of delayed rectifier K channels in rat heart and brain that cDNAs from members of this family can be used to screen a human heart cDNA library for IKr. We will test this hypothesis by cloning cDNAs that hybridize to the rat heart delayed rectifier K channel and expressing them in Xenopus oocytes. The rapid inactivation and rectification properties of this channel are primary determinants of the amount of current which can flow through the channel, and hence, the degree to which this current affects the shape and duration of the cardiac AP. We also propose that inactivation and rectification can be selectively altered by certain class III antiarrhythmic drugs. This hypothesis will be tested by examining the biophysical properties of the clone channels using two-electrode voltage clamp and macro-patch clamp techniques to determine which clones possess physiological and pharmacological properties that correspond to IKr. Clones that produce IKr channels will be sequenced and subjected to site-directed mutagenesis experiments to identify critical regions of the protein that are involved in rapid inactivation and rectification. Normal and altered clones will be characterized in the presence and absence of specific antiarrhythmic agents to determine their effect on inactivation and rectification. Information from these experiments may be useful in the design of new antiarrhythmic drugs with greater efficacy and improved safe.